Continuously variable ratio transmissions are well known in the art. Such transmissions are also referred to as continuously variable transmissions, CVTs, stepless variable transmissions, or infinitely variable transmissions. These transmissions have a gear ratio which may be continuously varied over some range, rather than shifted among several fixed ratios. Early CVTs included a rubber belt and pulley system, which was later upgraded to a steel belt for improved durability.
Other continuously variable transmissions use planetary gear sets, but large motors are used to control the rotational speed ratios. U.S. Pat. No. 6,761,658 describes an infinitely variable transmission which includes three planetary gear assemblies controlled by hydrostatic drives.
U.S. Pat. No. 4,973,295 describes a worm-planetary continuously variable transmission where the revolution of the planet gears is controlled by a worm gear assembly. In this transmission, the input and output shafts rotate in opposite directions. An additional gear assembly is required to reverse the direction of the output shaft rotation, decreasing the efficiency of this transmission.
U.S. Pat. No. 2,859,641 to Gleasman describes a worm gear assembly which is self-locking at gear ratios as low as 1.86:1. This type of assembly is commonly used in automobile differentials. Such differentials are referred to as ‘torque sensing’ or TORSEN (a registered trademark of JTEKT Corporation of Japan).